JPS5844716Y2 - protection circuit - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a protection circuit that protects speakers and the like from excessive output.

Conventional protection circuits of this type using relays have drawbacks such as the transistor and diode bias circuit that constitutes the relay drive control circuit uses a large number of resistors and it is difficult to set the bias voltage.

For example, in the conventional protection circuit shown in Fig. 1, the output from the midpoint voltage detection circuit A, which detects the offset voltage of the speaker terminal and transmits the level voltage that drives the relay, is output from the power transformer T and the rectifier circuit S. It is input to a muting circuit C which is supplied with power by a power supply circuit B.

Relay Y as a muting switch element drives a switch connected between the output amplifier and the speaker.

The output of the control transistor Q2 is applied to the base of the transistor Q1 that drives this relay Y via a Zener diode ZD.

The transistor Q2 has a control function of transmitting the detection voltage of the midpoint voltage detection circuit A and the discharge voltage through the diode D1 when the power is OFF to the transistor Q1, and turning it on and off.

Resistors R, R1, and R2 are bias resistors for the transistor Q2 in normal use, and the base and emitter of the transistor Q2 are connected to the + of the power supply circuit B including the smoothing capacitor through the resistor R1 and the resistor R2, respectively. connected to the side.

Further, the emitter of the transistor Q2 is grounded through a parallel circuit consisting of a resistor R3 and a capacitor C1, and is connected to the cathode of a Zener diode ZD.

Further, the anode of the Zener diode ZD is connected to the base of the transistor Q1.

The base of the transistor Q2 is further grounded through a parallel circuit of a capacitor C2 and a resistor R4 via the forward direction of the diode D1, and is also constituted by the secondary side of the power supply l/lance T and the diode D2 connected thereto. The capacitor C2 is charged to the + side through a half-wave rectifier circuit.

In such a conventional protection circuit, capacitor C1 and resistor R
2 determines the muting time when the power is turned on.

In addition, in order to properly reverse bias transistor Q2 so that it does not turn on during normal use, a resistor R is connected to its base.
The + side voltage of power supply circuit B is applied through 1.

The base of the transistor Q2 is grounded via the forward direction of the diode D1 by a parallel circuit of a capacitor C2 and a resistor R4.

Therefore, when the power switch 51 is ON, muting is applied for the charging time until the capacitor C1 becomes higher than the voltage between the Zener diode ZD and the pace emitter of the transistor Q1 via the resistor R2, and when the power switch S1 is OFF, the secondary side of the power transformer T is muted. The voltage quickly becomes zero voltage, the charge stored in the capacitor C2 is immediately discharged through the resistor R4, the base of the transistor Q2 becomes forward biased through the diode D1, the transistor Q2 becomes conductive, and the charge in the capacitor C1 is reduced. The transistor Q1 is quickly discharged and turned OFF, and the relay Y is turned OFF without a large delay, thereby preventing noise to the speaker.

Conventional protection circuits like this have many drawbacks, such as the large number of resistors used in the bias circuit for the transistors that make up the control circuit of the relay drive circuit, and the connections are complicated, as well as the difficulty in setting the bias voltage. Ta.

The present invention has been made to eliminate the above-mentioned drawbacks of the conventional technology, and provides a protection circuit that simplifies the bias circuit of the circuit that controls the relay drive circuit and facilitates bias setting by using a single diode. It is something.

The details of the present invention will be explained below with reference to embodiments and drawings.

In FIG. 2, the same reference numerals as in FIG. 1 indicate corresponding parts, and some of the explanations thereof will be omitted.

The difference from the conventional example in FIG. 1 is that the bias resistors R2 and R3 of the transistor Q2 are omitted, and one is connected to the base of the transistor Q2 and the other is connected to the diode D3 via the resistor R5 connected to the + side of the power supply circuit B. The emitter of the transistor Q2 and the capacitor C1 are connected so that a voltage is applied to them through the forward direction of the transistor Q2 and the capacitor C1, respectively.

The operation of the circuit of this embodiment configured in this manner will be explained.

When the power switch S1 is turned on, the capacitor C1 is charged through the path of resistor R5, diode D3, and capacitor C.

At this time, the base and emitter of the transistor Q2 become reverse biased due to the voltage drop across the diode D3, and the transistor Q2 does not conduct.

Therefore, capacitor C1 is composed of resistor R5, capacitor C
The potential increases with a time constant determined by 1, and when it becomes higher than the sum of the Zener voltage of the Zener diode ZD and the base-emitter voltage of the transistor Q1, the transistor Q1 becomes conductive and turns on the relay Y.

Next, when power switch S1 is turned off, diode D2
The secondary voltage of the power transformer T, which was applied through the resistor R4, instantaneously becomes zero, and the charge in the capacitor C2 is rapidly discharged through the resistor R4.

At this time, since there is a large smoothing capacitor in the power supply circuit B, the supplied DC voltage does not instantaneously become zero.

As a result, diode D1 turns on and resistor R4,
Grounded by capacitor C2, the base of transistor Q2 is forward biased, transistor Q2 becomes conductive, and the charge on capacitor C1 quickly discharges through transistor Q2, making transistor Q1 non-conducting and causing the base of transistor Q2 to become conductive.
The state becomes FF.

When the midpoint voltage detection circuit A operates when the power switch 51 is ON, the base of the transistor Q2 becomes forward biased through the midpoint voltage detection circuit A, and the transistor Q2 becomes conductive, rapidly discharging the charge in the capacitor C1, and the transistor Q1 is made non-conductive, and relay Y is output to 0FFL.

Next, another embodiment will be explained with reference to FIG.

In the embodiment of FIG. 3, the resistor R5 in FIG. 2 is connected in parallel with the diode D1, and the voltage applied to the base and emitter of the transistor Q2 and the capacitor C1 is transmitted from the secondary side of the power transformer T through the diode D2. The other circuit connections are the same as those shown in FIG.

To explain the operation of the circuit of this embodiment, the power switch S
, when ON, the AC voltage on the secondary side of the power transformer T is rectified by the diode D2, smoothed by the capacitor C2, and the resistor R4, and then sent to the base of the transistor Q2 via the resistor R5, and further to the base of the transistor Q2 via the forward direction of the diode D3. is applied to the emitter of capacitor C1.

The remaining operations are the same as those in FIG.

As described above, according to the present invention, by adding a diode to the bias of the transistor in the control circuit of the relay driving transistor, the number of bias resistors is reduced and the circuit configuration is simplified. What used to be reverse biased now becomes only the forward voltage of the diode (approximately 0.6 V).

Furthermore, as is clear from the explanation in the text, a highly reliable protection circuit is constructed with a simple circuit.

Further, there is an advantage that the protection circuit can be constructed using only the positive power supply voltage.

[Brief explanation of the drawing]

FIG. 1 is a conventional protection circuit diagram, FIG. 2 is a circuit diagram of one embodiment of the present invention, and FIG. 3 is a circuit diagram of another embodiment. D3...Diode, Ql...Transistor that drives relay Y, Q2...Control transistor, Y...Relay as a muting switch element

Claims (1)

[Scope of utility model registration request] a switch connected between the output amplifier and the speaker;
a DC power supply including a first transistor whose collector side is connected to a coil of a relay that controls the switch and whose emitter is grounded; and a smoothing capacitor that supplies a DC voltage to the relay and the first transistor; a first capacitor connected between the base of the first transistor and ground; a connection point between the first capacitor and the base of the first transistor connected to an emitter and a collector connected to ground; a second transistor having a polarity opposite to that of the first transistor; one end is connected to the base of the second transistor and the other end is connected to the emitter, and in the forward conduction state, the base of the second transistor
a first diode that provides reverse bias between the emitters;
a second diode, one end of which is connected to the base of the second transistor, and which applies a forward bias between the base and emitter of the second transistor in a forward energized state; and a second diode in parallel between the second diode and the ground. A third diode rectifies an AC power source that supplies a DC voltage to a connection point between the connected first resistor and second capacitor, and the second diode, first resistor, and second capacitor. A circuit that produces a half-wave rectified output with the same polarity as the DC voltage, one end of which is connected to the DC power supply or the connection point between the second diode, the second capacitor, and the first resistor, and the other end of which is connected to the connection point of the DC power supply or the second diode, and the second capacitor and the first resistor. and a second resistor connected to the base of the second transistor.